For those who stumble upon this in the future, and those who don't know about it yet:
Latest SourceMod 1.9 builds now support two new functions in order to detect if a translation phrase exists. Here's a snippet from the include.
PHP Code:
/**
* Determines if the specified phrase exists within the plugin's
* translation cache.
*
* @param phrase Phrase to look for.
* @return True if phrase exists.
*/
native bool TranslationPhraseExists(const char[] phrase);
/**
* Determines if a there is a translation for the speicifed language.
*
* @param phrase Phrase to check.
* @param language Language number.
* @return True if translation exists.
*/
native bool IsTranslatedForLanguage(const char[] phrase, int language);